The present invention relates to the method and apparatus to form fine irregularity densely onto the metal surfaces using laser beam. For instance, the present invention may be utilized to give graceful, fluorescent and reflective gloss as patterns or their likes to a part or all of the surfaces of metal accessories, domestic metal electric appliances, industrial metal products and various metal products.
Laser beam is a coherent light having a constant wave length including the same phase and is excel in its directivity. Laser beam can be converged by lenses to concentrate its high energy in a fine spot and has recently been applied to metal cutting, perforating (drilling), welding and so forth.
However, the previous art of the metal process by laser beam bases on the use of the maximum energy density at a focal point to fuse and evaporate metal in a beam spot radius instantly.
Even though a laser beam illuminates as a perfect parallel light from a laser oscillator, it is extremely difficult to condense the laser beam in the order of magnitude of its wave length due to the spreading of the beam by diffraction, the limitation of optical accuracy for light path and the minimum spot radius (several to 10 .mu.m) which can be converged by condensers. Therefore, the fine irregularity with the depth less than 1 .mu.m could not be formed on metal surfaces by the previous art of laser technique.
The demand of stainless products has increased in various fields due to the advantages of non-rusting characteristics, mechanical strength and giving a kind of heavy and thick feeling. As their surfaces also give their cold feeling by their original and metal color, various patterns have been tried to be formed by coloring the metal surface while keeping its natural metal gloss more or less. The following method was one of the representative techniques for coloring process. For instance, the surface of a stainless material is masked by a certain synthetic resin and the mask thus formed is taken away by chipping in many stripes' patterns and then immersed in a coloring solution to stain chemically the portion except the masked portion. Subsequently, the remaining masking material is removed away from the proccessed metal surface.